1. Technical Field
The present invention relates to a fuel injection device for a diesel engine.
2. Description of the Earlier Technology
Conventionally, there has been existing a fuel injection device for a diesel engine of such a type that conducts a preliminary injection and a main injection directly to a combustion chamber. This device performs the preliminary injection prior to the main injection so as to improve the combustion.
Patent Public Disclosure No. 10-18940 discloses one of the fuel injection devices of this type. This device begins the preliminary injection at an initial time of an intake stroke after an exhaust valve has closed to form in a combustion chamber a diluted fuel-air mixture, into which the main injection is carried out. This device performs the preliminary injection to form the diluted fuel-air mixture, thereby shortening the ignition lag of a main injection fuel. Thus it attempts to reduce NOx and exhaust amount of black smoke.
The above conventional technique begins the preliminary injection at the initial time of the intake stroke after the exhaust valve has closed. Therefore, it has the following problems.
During this term, intake air is rapidly introduced to lower the temperature of a gas within the combustion chamber quickly. If the preliminary injection is performed to this gas of low temperature within the combustion chamber, a preliminary injection fuel does not vaporize soon and therefore adheres to an inner surface of the combustion chamber as it remains liquid. This liquid preliminary injection fuel commences to vaporize with compression heat produced at a compression stroke. However, it cannot completely vaporize only with the compression heat. Accordingly, before it becomes a preliminary fuel-air mixture and spreads over the whole combustion chamber, the main injection is performed. Thus, needless to say, the aimed effect cannot be attained. Besides, unburnt fuel or gas increases in exhaust amount to deteriorate consumption accordingly.